Global Cooling Chilled Super-Hot Oceans of Early Earth, Study Finds

Scott Norris
for National Geographic News

October 25, 2006

Climate change on a grand scale occurred over much the Earth's early history, researchers say.

New geological data confirm the theory that complex life emerged only after a period of global cooling that lasted over three billion years.

Evidence that the world's oceans were once steamy cauldrons comes from the distribution of the element silicon in ancient rocks known as cherts.

In today's edition of the journal Nature, François Robert and Marc Chaussidon report that cherts formed from ocean sediments provide a kind of "paleo-thermometer" for seas during the Precambrian era.

The Precambrian is the vast stretch of time between Earth's formation more than 4.5 billion years ago and the rise of multicellular organisms about 600 million years ago.

The French researchers discovered that Precambrian ocean temperatures can be deduced from the ratios of different silicon variants—known as isotopes—found in chert layers that formed on the ancient seafloor.

The scientists found that the heaviest of the three naturally occurring silicon isotopes is more common in rocks formed in cooler water.

"We found that these Precambrian cherts have very peculiar isotopic compositions," said Chaussidon, of France's Center for Petrographic and Geochemical Research in Vandoeuvre-lès-Nancy.

"We interpret these as reflecting a global cooling of the oceans from temperatures perhaps as high as 158 degrees Fahrenheit (70 degrees Celsius) 3.5 billion years ago to 68 degrees Fahrenheit (20 degrees Celsius) at the end of the Precambrian."

"Runaway Greenhouse" Effect

Previous studies of Precambrian cherts have also concluded that the seas in which early life evolved were far hotter than those today.

But critics have maintained that those studies, which focused on oxygen isotopes, might have been compromised by more recent geological processes that altered the geologic record.

(Read related story: "Hell on Earth? Scientists Debate Planet's Early Years" [August 9, 2006].)

"This [new study] adds a major body of evidence and allows greater confidence in previous results," commented Paul Knauth, a geologist at Arizona State University in Flagstaff who conducted some of the earlier oxygen isotope studies.

"The large changes [in silicon isotopes] were quite surprising and were cleverly related to climatic temperatures," Knauth added. "It's a remarkable data set."

The French researchers say that the record based on ocean temperature is indicative of the changing climate of early Earth.

Early in the Precambrian, Earth received less heat from the sun, which is thought to have been 25 to 30 percent less luminous than it is today.

But high temperatures were maintained by an atmosphere extremely rich in greenhouse gases such as methane and carbon dioxide (CO2).

"If the temperature trend is correct, it appears that the early Earth almost went into a runaway greenhouse [effect], like Venus," Knauth said.

The Temperature of Life

Though inhospitable by today's standards, the hot Precambrian oceans were the setting for billions of years of biological evolution.

(See a National Geographic magazine feature about life that thrives near deep-sea hydrothermal vents.)

Single-celled organisms flourished in the ancient seas, and scientists believe some helped drive the process of global cooling.

Microbes capable of carrying out photosynthesis slowly reduced the amount of heat-trapping CO2 and helped create today's more temperate, oxygen-rich atmosphere.

Lower temperatures helped spur the emergence of complex plant and animal life at the end of the Precambrian.

Among the new life-forms at that time were abundant marine organisms that used silicon dissolved in seawater to build hard shells.

That development, Chaussidon says, spelled the end of the silicon-based temperature record, as marine life began to use more of the mineral, causing silicon levels in the sediments to drop.

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